XRD and Rietveld Quantitative Phase Analysis of Radio Frequency Suspension Plasma Sprayed Nano-hydroxyapatite Powders

Abstract

Abstract Ultra-fine hydroxyapatite powders were successfully synthesized using radio frequency (RF) suspension plasma spraying (SPS). This novel technique utilises the inherent characteristics of the RF plasma to axially feed and spheroidise a liquid suspension to produce spherical ultra-fine HA powders. This offers an alternative approach over conventional D.C. and flame spheroidising techniques which are better suited for solid feed stocks. Rietveld analysis was subsequently applied using Rietquan Quantitative Analysis software package to determine the amount of decomposed phases and amorphous content of the as-sprayed powder. This was also compared against quantitative XRD analysis employing internal and external standards. However, pure phases needed for calibration is scarce and amorphous calcium phosphate (ACP) is virtually impossible to isolate. In addition, the long and laborious task of obtaining calibration curves makes this technique unpopular. Nevertheless, conventional quantitative phase analysis (QPA) was carried out, using relative peak height ratios of HA and the phase involved, but the calculated decomposition only shows relative trends for a particular parameter variation. Determining the actual phase content is critical because of possible variations in biological responses when used as coatings and inserts in restorative orthopaedic implants. Varying tissue responses can arise from decomposed phases such as α and β-tricalcium phosphate (TCP) and tetra-calcium phosphate (TTCP) as well as ACP which generally have higher solubility as compared to crystalline. QPA via the Rietveld method provides a powerful tool that offers the user simultaneous quantitative phase determination of multiphase systems containing amorphous content. Unlike XRD QPA, the amorphous content could be indirectly calculated using crystalline alumina standard. XRD QPA results showed that decomposition generally rose with plate power without considering the amorphous content. With Rietveld QPA, the results showed an initial rise in decomposition before decreasing at higher plate powers. The amorphous phase content was calculated at different plate powers and concentration of suspension with the aid of alumina as an external standard. Results showed that the amorphous content increased substantially at higher powers. This study demonstrates the ability of Rietveld analysis to completely quantify all associated amorphous and crystalline phases within a multiphase system for any thermally treated material.